This application claims priority under 35 U.S.C. §119 to German patent application no. 10 2010 010 506.6, filed Mar. 6, 2010 in Germany, the disclosure of which is incorporated herein by reference in its entirety.
The disclosure relates to an electrohydraulic pressure control arrangement for controlling the pressure of a pressure medium, and to a method for pressure control. The electrohydraulic pressure control arrangement comprises a proportional pressure valve and a control loop structure. The control loop structure has a control path through which an actuating variable is fed to an actuating input of the proportional pressure valve starting from a setpoint pressure value. A pressure sensor detects an actual pressure value at a connection of the proportional pressure valve. A system model of a pressure valve assigns an estimated pressure value to the setpoint pressure value, and a subtraction element determines a control error as the difference between the estimated pressure value and the actual pressure value.
A pressure control arrangement of this kind for a proportional pressure limiting valve without or with integrated electronics is known from the company publication Rexroth, Boschgroup RD 29162, dated August 2007. Different characteristic curves between the pressure in the connection region P and the volumetric flow rate in 1/min for different pressure stages are also published in this company publication, with the characteristic curves clearly showing that, in the case of a real proportional pressure valve, the pressure in the connection P is dependent on the volumetric flow rate through the pressure valve. In the case of an idealized system model however, the pressure in the connection P is independent of the volumetric flow rate through the valve, and this means that, in contrast to the real proportional pressure valves, the characteristic curve runs horizontally in the ideal case.
Furthermore, a dissertation from the Department of Control Engineering, Friedrich-Alexander University, Erlangen-Nuremberg, Identification of Parameters of a Pressure Control Valve using Algebraic Methods, by Daniel Kotzian, 2007, discloses that setpoint pressure characteristic curves of a real proportional pressure valve disadvantageously exhibit, despite a dither, pressure hysteresis which is undesirable for operation of proportional pressure valves, and therefore an additional controller, which is intended to maintain the setpoint pressure at different volumetric flow rates, should be provided. However, in the event of large control errors, there is a risk of any disturbances or oscillation stimuli, in particular when the pressure which is actually reached deviates to a great extent from the ideal horizontal P-Q characteristic curve as a function of the quantity of fluid which is intended to be delivered or discharged in the case of the known additionally controlled pressure limiting valve. This error is the result of hydromechanics in the valve and can, in extreme cases, lead to stimulation and build-up of oscillations in the pressure control arrangement.
Known controller structures of this kind can, for all intents and purposes, be optimized only in response to small signal behavior, and for this reason a disadvantageous control loop behavior is often established at high setpoint value amplitudes. This disadvantageous control loop behavior is usually counteracted using control loop dynamics which are set to be low. A further option is to mask large control errors for the controller by a correspondingly limited passband. However, this presents the problem that the passband has to be selected to be very large for reliable functioning of the control loop over the entire operating range, as a result of which the intrinsically positive effect of such a passband is attenuated again.
The object of the disclosure is to provide an electrohydraulic pressure control arrangement with improved control loop behavior for proportional pressure valves, and a method for pressure control using the electrohydraulic pressure control arrangement.
This object is achieved by way of the subject matter set forth herein. Advantageous developments of the disclosure are set forth herein as well.
The disclosure provides an electrohydraulic pressure control arrangement for controlling the pressure of a pressure medium, and a method for pressure control. The electrohydraulic pressure control arrangement comprises a proportional pressure valve and a control loop structure. The control loop structure has a control path through which an actuating variable is fed to an actuating input of the proportional pressure valve, starting from a setpoint pressure value. A pressure sensor detects an actual pressure value at a connection of the proportional pressure valve. A system model of a pressure valve assigns an estimated pressure value to the setpoint pressure value, and a subtraction element determines a control error as the difference between the estimated pressure value and the actual pressure value. A weighting element subjects the control error to a weighting operation and thus determines a weighted control error. A controller modifies and modifies a signal on the control path, in the sense of minimizing the weighted control error, during operation on the basis of the weighted control error. The weighting element is designed such that relatively large control errors are attenuated to a greater extent than relatively small control errors.
This electrohydraulic pressure control arrangement has the advantage that the controller compensates the control errors by means of a feedback branch and the control errors are weighted by means of a weighting function during the feedback operation. In this case, the passband of the weighting function can selectively be permanently set or automatically tracked as a function of the volumetric flow rate and by means of a volumetric flow rate estimation of the actual valve throughflow rate from an actual controller output signal and an actual pressure value. The electrohydraulic pressure control arrangement according to the disclosure and the method for pressure control improve the control loop behavior primarily in the large signal range in the case of pressure valves with pressure control. An advantageous solution with which, for example, reverberation is avoided, primarily in the large signal range, is provided by calculating the difference between an estimated pressure value and an actual pressure value and by weighting by means of an implemented characteristic map or an approximating function as a control variable for the required passband. To this end, adapting the passband ensures that the control loop function is always carried out with the minimum possible passband, and so the desired improvement in the control loop behavior is determined.
In a preferred embodiment of the disclosure, the volumetric flow rate estimation is realized as a characteristic map or by a function which approximates the characteristic map. A function of this kind can be represented, for example, for the estimated volume value QS by the following equation:QS(UR,PA)=a·UR+b·PA+c, where a, b and c can be coefficients relating to the controller output signal UR, the actual pressure value PA and relating to the estimated volume value QS. In this case, this function or else the characteristic map is matched to the respective proportional pressure valve.
In a further embodiment of the disclosure, an anti-windup block is provided in the pressure control arrangement, said anti-windup block limiting an I component of the controller which may be present when actuating signal limitations are present. To this end, the anti-windup block has a limiter and a proportional element which is connected downstream via a coupling point, the output signal from said proportional element being connected to the PI controller input via a further subtraction element. This embodiment effectively prevents the I component of the controller from “running away” by means of actuating signal limitations.
A method for pressure control in an electrohydraulic pressure control arrangement which has a proportional pressure valve, which reacts to a prespecified setpoint pressure value, comprises the following method steps.
First of all, an actuating variable UR is fed to an actuating input of the proportional pressure valve via a control path starting from a setpoint pressure value UPset. An actual pressure value is detected at a connection of the proportional pressure valve by means of a pressure sensor. Furthermore, the setpoint pressure value is assigned to an estimated pressure value by means of a system model of the pressure valve.
A control error in the form of the difference between the estimated pressure value and the actual pressure value is then determined by means of a subtraction element and the control error is subjected to a weighting operation in order to form a weighted control error. Furthermore, the weighted control error is minimized by modifying a signal on the control path by means of a controller, with a weighting element being designed such that relatively large control errors are attenuated to a greater extent than relatively small control errors.
A characteristic map or a function which approximates the map of characteristic curves can be stored in order to estimate a setpoint volumetric flow rate. An I component, which may be present, of a controller can be limited by an anti-windup block in the pressure control arrangement when actuating signal limitations are present. It is also possible, in the anti-windup block, for a proportional element to be connected downstream of a limiter via a coupling point, the output signal from said proportional element being fed to a controller input via a further subtraction element.